1. Field of the Invention
The present invention relates to a perforation detector of a camera for detecting a perforation of a film to control a feed amount of the film.
2. Related Background Art
When a film is to be fed by a predetermined amount such as in initial film feed or normal film feed, the film can be exactly positioned by detecting when a perforation formed in the film reaches a predetermined position and stopping the film feed at that moment.
A photo-reflector has been known as a perforation detector for this purpose. The photo-reflector comprises a light projection unit (for example, a light emitting diode) which projects a light to the film and a photo-sensor (for example, a photo-diode) which senses reflected light from the film, and detects the perforation from an output of the photo-sensor. Namely, when the perforation is not positioned at an area to which the light from the light projection unit is projected, the reflected light intensity from the film is high and the photo-sensor senses a higher light intensity, and when the perforation is positioned at the area to which the light is projected, the reflected light intensity from the film is low and the light intensity sensed by the photo-sensor is low. Accordingly, whether the perforation is at the predetermined position or not can be detected from the output of the photo-sensor.
However, the reflection index of the film varies from maker to maker or from sensitivity to sensitivity. Since the prior art photo-reflector has a fixed light intensity of the light projection unit, the output of the photo-sensor of the photo-reflector varies with the reflection index. This causes the following problem.
FIG. 6 shows an output waveform of the photo-sensor to areas of the film when the light intensity of the light projection unit is constant, and a pulse signal waveform derived by reshaping the output. FI denotes a film, G denotes a photographing screen and P denotes a perforation. S1 denotes a theoretical photo-sensor output when the reflection index of the film is sufficiently high, and S2 denotes a photo-sensor output when the reflection index of the film FI is low. As shown, S2 is lower than S1. Vref denotes a reference voltage to determine the presence or absense of the detection of the perforation. The perforation is detected when the photo-sensor output S1 or S2 is no higher than Vref. B1 and B2 denote pulse signals derived by reshaping the outputs S1 and S2, respectively. As seen from the figure, when the film is fed in the direction A, the pulse B2 is produced later than the pulse B1 for the same perforation. This means that the detection of the perforation is delayed when the reflection index of the film is low compared to that when the reflection index is high. In such a case, the film FI cannot be exactly positioned.
This problem may be solved by setting the light intensity of the photo-diode to a fairly high level, but if the light intensity is merely increased, it wastes battery power where the reflection index of the film is sufficiently high and the light intensity may be low. Accordingly, this in vain shorten a battery life.